This invention relates to optical performance monitoring.
Optical performance monitoring schemes are often used in fiber optic systems to measure channel power. As a signal propagates along the fiber, it is degraded and attenuated. To maintain the signal, periodically the signal is amplified. By measuring the signal power, the appropriate amount of amplification is chosen that optimizes operation of the fiber optic system.
In a wavelength division multiplexing (WDM) scheme, optical performance monitoring is even more demanding. In WDM, multiple signals are transmitted simultaneously by transmitting each signal in a separate channel (i.e. separate carrier wavelength). Fiber nonlinearities such as stimulated Raman scattering (SRS) can make such measurements difficult.
One way of measuring channel power uses optical spectrum analyzers at each amplifier site in a fiber optic system. These relatively expensive devices directly measure the optical power for each channel. This technique becomes quite expensive on long haul optical fiber systems where a large number of amplifier sites are needed.
Another scheme applies a unique dither tone to each channel. The dither tone has amplitude that is a known fraction of the launch power in the channel. By measuring the tone power at some subsequent point in the fiber, the channel power at that point is inferred. This scheme avoids the relatively high costs of an optical spectrum analyzer but the dither tone scheme is susceptible to errors induced by fiber nonlinearities.